The present invention relates to the transfer of subscriber identity in a protected format in a telecommunications network, particularly in mobile communications systems.
Protecting subscriber identity means the concealment of the identity of a user of a telecommunications network from outsiders. Protection of identity is of special importance in mobile communications systems, where the subscriber and the network identify themselves to each other before the connection is made. If subscriber identity is transferred unprotected, it is possible to follow the movements of the subscriber by monitoring the radio connections established between the subscriber and the network. In addition, by protecting the subscriber""s identity it is possible to considerably complicate the deciphering of data communications. Protection of subscriber identity may be desirable in fixed network systems as well. In circuit-switched systems of a fixed telephone network, the subscriber identity is determined by the subscriber line, and thus subscriber identity is not transferred as a message; instead, it is always determined by the subscriber line used. In packet-switched systems, the subscriber identity is transferred in each data packet sent by the subscriber, and so the subscriber identity can be fully concealed using encryption methods suitable for use in mobile communications systems.
FIG. 1 illustrates a known mobile communications network. The figure shows two mobile services switching centres MSC1, MSC2, base station controllers BSC, base transceiver stations BTS, a mobile station MS, a home location register HLR, and an authentication centre AUC typically located in association with a HLR. The mobile services switching centres are capable of establishing signalling connections with the home location register HLR and the authentication centre AUC.
Each mobile subscriber has a home public land mobile network HPLMN operated by an operator with which the subscriber has concluded an agreement. The user""s subscriber data is stored in the home location register HLR of his home public land mobile network and the related authentication centre AUC. The authentication centre has all the data necessary for verifying the authenticity of the identity communicated by the user. In the home location register HLR, the mobile subscriber international ISDN number MSISDN can be linked to the user""s international mobile subscriber identity IMSI. In addition, information on the services ordered by the subscriber as well as the user""s current location to an accuracy within the visitor location register VLR address is stored in the home location register. No subscriber can be registered with more than one visitor location register VLR at any given time.
The visitor location register VLR located in association with the mobile services switching centre MSC is also used to maintain data on the location of users registered with the applicable visitor location register to an accuracy of a so called location area. In addition to the services offered by the home public land mobile network HPLMN, a subscriber can use the services available in those other visited public land mobile networks VPLMN with which his own operator has signed a roaming agreement.
Through the mobile services switching centres MSC, mobile communications systems are linked to fixed telephone networks, such as a public switched telephone network PSTN or an integrated services digital Network ISDN. Several base transceiver stations BTS are connected to a base station controller BSC. The base transceiver stations are capable of making connections with mobile stations MS consisting of mobile equipment ME and subscriber identity modules SIM using channels of the so called air interface.
In mobile communications systems representing prior art, the objective is to transmit subscriber identity protected across the air interface. For example, the known GMS system uses a temporary mobile subscriber identity TMSI illustrated in FIG. 2 to conceal the user""s international mobile subscriber identity IMSI.
As shown in FIG. 2, information about the temporary mobile subscriber identity TMSI is only stored in the user""s visited location register VLR and mobile station MS. When the network and the mobile station contact each other, the temporary mobile subscriber identity, if available, is always used for identification instead of the international mobile subscriber identity IMSI. TMSI consists of two components, one being the location area code LAI and the other the temporary subscriber identity code TIC (TMSI Code) that uniquely identifies the user within the location area. The TIC code is unique within one location area LAI. Information about the temporary mobile subscriber identity TMSI is not transmitted to the home location register HLR; instead, the temporary mobile subscriber identity TMSI used across the air interface is always converted in the visited location register VLR into the international mobile subscriber identity IMSI. For communications between the home location register HLR and the visited location register VLR, the permanent identification IMSI is always used for subscriber identification purposes.
FIG. 3 illustrates the generation and maintenance of the temporary mobile subscriber identity TMSI. VLR assigns a mobile station a new temporary identity, for example in connection with each location update. The mobile station sends to the network a non-encrypted LOCATION UPDATE REQUEST 301 to identify itself using the temporary mobile subscriber identity TMSI, if defined, and communicates its previous location area. The request must be transmitted non-encrypted because the network has no previous information on the user""s identity or user-specific encryption keys. The request is forwarded to the visited location register VLR. When receiving the request, the visited location register requests the necessary information from the user""s previous visited location register on the basis of the previous location area data. At this point, the network directs the mobile station to activate cipher mode (phase 302, CIPHER MODE COMMAND) and the mobile station acknowledges the command (phase 303, CIPHER MODE COMPLETE). The network indicates acceptance of the location update (phase 304, LOCATION UPDATE ACCEPT) and gives the user a new temporary mobile subscriber identity TMSI (305, TMSI REALLOCATION COMMAND), in response to which the mobile station acknowledges the new identity (306, TMSI REALLOCATION COMPLETE). The new TMSI can also be incorporated in the phase 304 message LOCATION UPDATE ACCEPT, in which case the phase 305 TMSI REALLOCATION COMMAND is not used.
Where possible, the GSM system always uses the temporary mobile subscriber identity TMSI that conceals the subscriber""s true identity. To ensure that TMSI can be used, it must be possible to link it to the international mobile subscriber identity IMSI in the visited location register VLR. However, this is not possible when the user contacts the network for the first time. Additional problems are created by situations where VLR, due to loss of data caused by a malfunction, is incapable of linking the temporary mobile subscriber identity TMSI to the international mobile subscriber identity IMSI. For this reason, the network may always ask the mobile station to send the original IMSI, which will then, in response, be transmitted in a non-encrypted format by the mobile station.
Another known method for protecting user identity in transmission is the technique used in the TETRA system. Similarly to the GSM system, the TETRA system may employ an encryption procedure called alias short subscriber identity ASSI, which is based on temporary identity. In addition to, or instead of, ASSI, TETRA may also use encrypted short identity ESI, which is described in greater detail in the ETS 300 392-7 Specification published by ETSI (European Telecommunications Standards Institute).
Generation of the encrypted short identity ESI is illustrated in FIG. 4. The encrypted short identity ESI is computed using the algorithm TA61 and the SSI identity and the common cipher key CCK that is common to several users, or the static cipher key SCK, as input data.
The static cipher key SCK is always used before the authentication process is carried out. A maximum of 32 static cipher keys, to be identified by the identification number SCKN (SCK Number) sent by the network to the mobile station, can be associated with any single subscriber identity. A common SCK is assigned to all those mobile stations in a single switching and management infrastructure SwMI that have access to the network section concerned.
The common cipher key CCK assigned to several users and, if necessary, changed by the network, is sent to the subscribers in a format encrypted with the derived cipher keys DCK provided by the authentication process, which means that it cannot be used until authentication is completed. Only one common cipher key CCK may be operative in the location area at any given time. As it is, all the mobile stations in the same location area can decipher any identity encrypted with the common cipher key CCK.
The problem of the arrangement described above is that several mobile stations know the cipher key and can thus decipher identity. Since all changes to the cipher key are always done by the network, the same encrypted short identity ESI is typically used several times. Third, the network with which the mobile station communicates, learns the identity of the mobile subscriber even when it does not need to know it.
The objective of the present invention is to eliminate these problems associated with prior art. This objective is achieved by using the method described in the independent patent claims.
The idea of the invention is to encrypt the subscriber identity data to be sent to the transmission network using a cipher key common to a certain group of users and a random number which is sent to the network attached to the encrypted identity data. Such a group of users may consist of all the subscribers of a given operator, all the users in one home location register, or any group of users defined within one home location register.
The transmission network must be capable of routing the message containing the identity of the subscriber to the subscriber""s home public land mobile network or other network defined as reliable. Therefore, the network must be able to identify the subscriber with the accuracy necessary for routing, such as to within the subscriber""s home network or home location register. It is advantageous to protect the identity without, however, encrypting the component identifying the user""s home public land mobile network.